Ch. 12] REVETMENTS 223 



alignment of the river and related considerations. The revetment con- 

 stitutes a protective coating laid on the bank to eliminate or reduce 

 loss of material due to the river's attacking the bank. Revetments 

 have been quite successful in this respect, but it is apparent that a 

 revetment in itself has no structural strength and will be carried away 

 if the bank on which it is laid is unstable. Thus, important engineering 

 problems are involved in the determination of the stability of a bank 

 when subject to scour and seepage forces. The determination of a safe 

 slope to which the bank can be graded is important, for the cost of the 

 work involved is greatly dependent on this slope. Construction prob- 

 lems increase greatly if the bank becomes flat, and slopes flatter than 

 1 on 3 or 1 on 3.5 are difficult to revet, from the construction view- 

 point, and very expensive,' from the economic viewpoint. Conse- 

 quently, low factors of safety must often be used in determining a sat- 

 isfactory slope to which the bank may be graded for placement of 

 revetment. As a matter of interest, the Mississippi River Commission 

 has under investigation at the Waterways Experiment Station a com- 

 bined hydraulics and soils investigation of factors affecting the loca- 

 tion, design, and construction of revetments. This includes an an- 

 alysis of reasons for failures which may be due both to the large 

 hydraulic forces involved and to the physical properties of the mate- 

 rials, such as shear strength, resistance to seepage forces, structure, 

 density of sand deposits, and susceptibility to flow slides. 



In the northern and central portions of the valley, revetments are 

 important in protecting sandy river banks against scour. Seepage 

 forces due to the natural draining of water out of the banks or to 

 rapid fall in river stages become important, especially if the soils are 

 not free-draining. Many drainage methods have been investigated by 

 the Mississippi River Commission, and horizontal drain wells are now 

 being considered. 



In the central portion of the valley, banks often consist of a top 

 stratum of silts and clays 20 to 40 feet thick overlying deep beds of 

 highly pervious sands and gravels which are readily attacked by action 

 of the river, whereas the top stratum is more resistant. However, the 

 top stratum may have a low shearing strength; then the problem be- 

 comes complex, because the revetment must be laid on a bank, which 

 must be graded to a slope that is stable, and must protect under- 

 lying sandy sediments which are easily scoured out. 



In the southern portion of the Alluvial Valley the surface clays ex- 

 tend to such depth that protection of silty sediments against attack 

 by the river becomes of less importance. As a result of the resistance 

 of these clays to erosion, the tendency of the river to meander is much 



